Metal Story: Chromium (Cr) – The “Red Lead” Mystery
Among the numerous designation of steels in any Soviet metallurgical handbook one is bound to come across many that include the symbol “X”: X18H10T, X12M, 0X23I-05, IIIX5, 8X4B401, X147=14H3T, 12X2HBOA, 30XMIOA, etc. For the uninitiated this “cypher” is as mysterious as hieroglyphs. But is steelworker sees his way in these seemingly random combinations of letters and figures as easily as a musician reads his score. Even a cursory glance at these designations is enough to notice the common element in them: they all contain certain quantities of the element chromium (as indicated by the letter X – the Russian for Cr).
Along with its alloying “colleagues” – nickel, tungsten, molybdenum, vanadium, titanium, zirconium, niobium, etc. – chromium makes it possible to produce steels designed for a multitude of purposes. Steel used in modern technology must “know” many things: how to resist colossal pressures and chemical “aggressors”, endure lasting overloads, yield to machining and resist heat and cold. Chromium has its own part to play in “teaching” steel all these wonderful properties.
Back in 1776 I.G. Leman, a St. Peterburg professor of chemistry, described a mineral that had been found in the Berezov mine, 15 kilometres from Ekaterinburg (today Sverdlosk) in the Urals. Treating the mineral with hydrochloric acid the chemist with a white precipitate in which he discovered lead. A few years later (in 1770) Academinian P.S. Pallas described the Berezov mines: “The Berezov mines consist of four pits that have been worked since 1752. Apart from gold, there is silver and lead ores mined there, and there also occurs a remarkable red lead mineral which has not been discovered in a single other mine in Russia. This ore comes in different colors (sometimes it looks like cinnabar), is heavy and semitransparent. Sometimes small, irregular pyramids of this mineral are imbedded in quartz like little rubies. When ground to powder, it makes a good yellow pigment…” This mineral was given the name “Siberian red lead”. Subsequently it came to be called “crocoite”.
At the end of the 18th century Pallas brought a specimen of crocoite to Paris. In 1796, the well-known French chemist Luis Nicolas Vauquelin, who had become interested in the mineral, made a chemical analysis of it. This is what he wrote about the results: “All specimens of this substance that are kept in several mineralogical rooms in Europe had been brought from this (Berezov – S.V.) gold mine. It used to be very rich in the mineral, but now they say that several years ago its reserves became exhausted and that today it is bought at the price of gold, particularly if it is yellow. Samples of this mineral that are of irregular shapes or broken into pieces can be used in painting where they are appreciated for their orange-yellow tint which does not change in the air… The beautiful red color, transparency and crystalline form of the Siberian red mineral are the properties that have aroused mineralogists’ interest in its nature and the place where it comes from. Its substantial specific weight and the accompanying lead ore provide every reason to suppose that the mineral contains lead…”
In 1797 Vauquelin repeated his analysis. He boiled powdered crocoite in a potassium carbonate solution. The experiment yielded lead carbonate and a yellow solution which contained the potassium salt of a then unknown acid. When mercuric salt was added to the solution, a red sediment was added to the solution, a red sediment was formed and when it reacted with lead salt, it deposited a yellow sediment, and when stannous chloride was introduced, the solution turned green. After lead had been precipitated by means of hydrochloric acid, Vauquelin evaporated the filtrate and mixed the red crystals which had been formed (they were chromium anhydride) with carbon, places it in a graphite crucible and heated to a high temperature. When the experiment was over he discovered in the crucible countless fused grey metal needles that weighed only a third of the weight of the starting material. Thus a new element was obtained. A friend of Vauquelin’s advised him to give it the name “chromium” (from the Greek “chroma” meaning “color”) because of the bright and diverse coloring of its compounds. As a matter of fact, the component “chrom” is found in many words not associated with the element chromium. For example, the word “chromosome”, translated from the Greek means “body which colors”, an instrument called “chromoscope” is used to obtain a color image; the popular photographic films bear the names “isopanchrom”, “panchrom” and “orthochrom”; astrophysicists term some bright formations in the atmosphere of the sun “chromospheric flares”, and so on.
At first Vauquelin did not like the idea of naming his metal “chromium” at all since it was of a modest grey color which did not seem to fit that name. But still his friends persuaded him to accept it and after the discovery had been officially registered by the French Academy of Sciences, chemists all over the world included “chromium” in the list of elements known to science.
Fortune apperaed to be well disposed to the new metal. Its high melting point, extreme hardness and its ability readily to alloy with other metals, iron in particular, aroused the liveliest interest of metallurgists. This interest has not dampened is the leading field in the consumption of chromium.
Chrominum possesses all the properties typical of metals; it is a good heat conductor, an excellent electrical conductor, and, like most metals, has lustre. One curious fact sets it apart, though: heated to a temperature of about 37 degree Celcius, it shows signs of “definance”. Many of its properties chande drastically, by leaps and bounds. Its internal friction reaches its maximum, while its modulus of elasticity drops to its minimum. A sudden change also occurs in its electrical conductivity, the linear expansion coefficient and thermoelectromotive force. So far scientists have been unable to explain these anomalies.
Even insignificant impurities render chromium very brittle which makes it practically impossible to be used as a structural material, but as an alloy component, it has always been favoured by metallurgists. Even the addition of a small quantity of chromium makes steel hard and wear-resistant. These characteristics are indispensable in ball-bearing steel, which includes, along with chromium (1.5%), also carbon (about 1%). The chromium carbides precipitated in this steel are exceedingly strong, which is what enables it to resist wear, one of the worst enemies of metals.
Stainless steel which had good corrosion and oxidation resistance contains between 17 and 19% of chromium and between 8 and 13% of nickel. But this steel is averse to carbon since excessive quantities of it are bonded into carbides precipitated on the boundaries of the steel grains, reducing the chromium content in the grains themselves and making it hard for them to resist the action of acids and oxygen. Therefore, stainless steel must not contain more than a minimum (0.1%) of carbon.
At high temperatures steel may become scaly. Some machine parts heat up to hundreds of degrees. To ensure that the steel from which the parts are made does not “suffer” from this malady an addition of 25 to 30% of chromium is suffficient. Such steel will withstand temperatures of up to 1000 degree Celcius.
Nichromes – alloys of nickel, chromium and iron – make good heating elements. The introduction of cobalt and molypdenum to these alloys enables the metal to withstand great loads in the 650-900 degree Celcius temperature range. From them the blades of gas turbines are manufactured. An alloy of cobalt, molypdenum and chromium (comochromium) is harmless for the human organism, hence its application in reparative surgery.
An American firm has recently developed some new materials in which magnetic in which magnetic properties change under the effect of temperature. According to scientists, these materials which are based on compounds of manganese, chromium and antimony will find application in many automatic devices sensitive to temperature changes. They will successfully replace more expensive thermoalloys.
Today the bulk of the world chromium ore output is turned over to ferroalloy plants producing a variety of ferrochromium alloys and metallic chromium.
Ferrochromium was produced for the first time in 1820 by reduction of ferric and chromic oxidies by charcoal in a crucible. Pure metallic chromium was obtained in 1854 by electrolysis of water solutions of chromic chloride. The first attempts to smelt carbonic ferrochromium in a blast furnace were made at about the same time. The first patent for chromium steel was issued in 1865. The demand for ferrochromium began to grow rapidly.
Electricity, or more exactly, the electrothermal method played a crucial role in the production of metals and alloys. In 1893 the French scientist Moissan smelted carbonic ferrochromium in an electric furnace which contained 60% of chromium and 6% of carbon.
In prerevolutionary Russia ferroalloy production developed at a snail’s pace. The plants in the south smelted miserly amounts of ferrosilicon and ferromanganese. In 1910 the Poroghi, a small electrometallurgical plant, was built on the river Satka (Southern Urals). It started the production of ferrochromium and later, ferrosilicon, but still it was too small to meet the national demand for these alloys and Russia had to import them almost wholly from other countries.
After 1917, the young Soviet state could not let itself be dependent on the capitalist countries in a field of such vital importance as the manufacture of high-grade steels which was the main consumer of ferroalloys.
Implementation of the great industrialization plans largely depended on the availability of steels – structural, tool, stainless, ball-bearing, motor and tractor. And these steels had chromium as one of their principal components.
Between 1927 and 1928 the planning and building of ferroalloy plants began. The Chelyabinsk ferroalloy plant, the first of its kind in the Soviet Union, was commissioned in 1931.
V.S. Yemelyanov, Corresponding Member of the USSR Academy of Sciences, one of the founders of the Soviet quality metals making industry, who during that period was in Germany studying the experience of metallurgical plants there, recalls this curious conversation in his memorirs:
“In 1933 I asked the chief engineer at a small German plant, “To whom do you sell ferrochromium made by this plant?”
“He began to enumerate: ‘Something like 5% of the output we sell to nearby chemical plants, the Becker plant buys 2%, some 3%…’
‘Does the Soviet Union buy much from you?’ I interrupted him.
‘Well, it depends. But usually we send 75% to 80% of our output to your plants. And, as a matter of fact, we work on the chromium ore from the Urals.’”
Indeed in those days Soviet chromium ore was exported not only to Germany, but also to Sweden, Italy and the United States. And that’s where we bought our ferrochromium.
That state of affairs came to an end when two ferroalloy plants were built in Zaporozhe and Zestafoni in 1933. Since then the Soviet Union stopped importing major ferroalloys and was soon even able to export them. The national industry had pratically fully been supplied with the ferroalloys it needed.
Addressing the 17th Congress of the Commuist Party of the Soviet Union, Sergo Ordzhonikidze, People’s Commissar for Heavy Industry, said: “…if we did not have high-grade steels, we would not have had the motor and tractor industries. The price of the high-grade steels we are now using is more than 400 million roubles. If we had to import them, we would have to pay 400 million roubles annually. That would, sure as well, have pushed us right back into the clutches of capitalism.”
In 1936 immense deposists of chromite, the main raw material for the production of ferrochromium were discovered near Aktyubinsk in Kazaskhstan. The Aktyubinsk ferroalloy plant which was built on the basis of this deposit during the war subsequently became a major enterprise putting out ferrochromium and chromium of all grades.
Abundant reserves of chromite are also found in the Urals. Among the most industrially important deposits are the Saranovsk, Verblyuzhyegorsk, Alapayevsk, Monetnaya Dacha and Khaliovsk. The Soviet Union holds a leading place in the the world in the explored reserves of chromium ores.
Outside the Soviet Union chromium ore deposits are situated in Turkey, India, New Caledonia, Cuba, Greece, Yugolsavia and in some African countries. Meanwhile there is absolutely no chromium in some advanced industrial countries, including Britain, France, Federal Republic of Germany, Italy, Sweden and Norway. The ores the United States and Canada have are so poor that they are practically useless as a ferrochromium raw material. Incidentally, the chromium content in the earth crust comes to 0.02%.
Chromites have a vast use in the refractory materials industry. A combination of magnesitie and chromium is excellent for the manufacture of firebricks used to line open-hearth furnace and other metallurgical equipment. Such bricks not only resist heat, but are also indifferent to repeated and drastic temperature changes.
In chemistry chromites are used for the production of the biochromates of potassium and sodium and of chrome alum used in tanning to make leather shiny and durable. Such leather is called “chrome leather”, hence, “chrome boots”, that used to be so highly regarded in Russia.
Justifying its name as it were, chromium is indispensable in the production of dyes for the glass and ceramics industry and for the textile industry.
Every evening the ruby stars of the Kremlin light up the Moscow sky. Among the precious stones ruby is second only to diamond. According to an old Indian legend, rubies are drops of blood shed by gods. This is how it tells about their origin: “Heavy drops of blood fell onto the bosom of the river, into the deep waters and the reflection of beautiful palm trees. And then the river began to be called Rawanaganga, and the drops of blood that had turned into ruby gems began to glow, and as darkness descended, they glowed with a magic fire which burned inside the gems and pierced the waters with its hot rays…”
Today the technique of making the beautiful red gem has become much simpler and gods do not have to shed their holy blood; it is replaced by aluminum oxide with an addition of chromium oxide (to this substance ruby crystals owe their beautiful color). But it is not merely their appearance that explains the value of artificial rubies: born with their helps, the laser works miracles like that magic ray created by engineer Garin’s “hyperboloid” and Alexey Tolstoi’s rich imagination. The laser is capable of cutting any metal as easily as scissors cut paper, or making the finest perforations in diamonds, corunda and other such “hard nuts”, completely ignoring their universal reputation for toughness.
Chromic oxide enables tractor manufacturers to reduce considerably the engine running-in time, an operation required in order to let the rubbing parts of the engine “get accustomed to each other”. It used to be a lengthy process to which tractor builders could not be reconciled. The way out was found when a new fuel additive containing chromic oxide was developed. The secret of its effect is simple: when chromic oxide burns it forms tiny abrasive particles, which, after settling on the inside walls of the cylinders and other surfaces subjected to friction, smooth out all rough spots, polish the components, and firmly adjust them to each other. Combined with the use of a new lubricant, this additive has made it possible to reduce running-in time to thrtieth of what it was before.
Not long ago chromium was given yet another “job”: American engineers manufactured and experimental magnetic tape, the working layer of which contained, instead of particles of ferric oxide, particles of chromic oxide, which dramatically improved the quality of recording and made the tape more reliable in operation.
Chromium “finds” itself a lot to do in many fields, including the manufacture of photographic materials, drugs and chemical catalysts and also in metal coatings. Chrome coatings deserve, we believe, a more detailed description.
It was noticed a long time ago that chromium was not only extremely hard (as far as this property is concerned, it has no rivals among metals), but also effectively resisted corrosion in the open air and did not react with acids. Attempts were made to coat articles from other metals with chrome electrolytically to protect them from corrosion, scratches and other “traumas”. It was found, however, that chrome coating was porous, easily peeled off and did not justify the hopes that had been pinned on it.
For nearly 75 years had scientists grappled with the problem of chrome plating and it was only in the 1920s that solution was found. The failure was explained by the fact that the chromium used in the electrolyte of the chrome bath was trivalent coating. It was found that it was six-valent chromium acid, whose chromium valence was six, began to be used as the electrolyte. The protective coatings (on some external parts of automobiles, motorcycles and bicycles) are up to 0.1 milimetre thick, but when used for decorative purposes (in the finish of clocks and watches, door knobs and other objects not seriously endangered by corrosion) they may be exceedingly thin – 0.0002 to 0.0005 milimetre.
Another method of chrome plating is the diffusion process, whereby the plating takes place in a furnace and not in the chrome bath. In the early stages of development of this process the steel part being coated was placed in chromium powder and then heated in a reducing atmosphere to a high temperature. A chromium-enriched layer was thus formed on the surface of the steel part. In hardness and corrosion resistance that coating was vastly superior to the steel from which the part was made. But in that process too some “buts” were discovered: At a temperature of about 1000 degree Celcius, chromium powder began to sinter, and besides, carbides preventing diffusion of chromium into steel precipitated on the surface of the metal which was being plated. Another chromium-carrier had to be found. The introduction of chloride or iodide, the volatile chromium haloids, made it possible to lower the temperature of the process.
Chromic chloride (iodide) is produced directly in the chrome-plating furnace by passing the vapours of a corresponding halogen acid through chromium or ferrochromium powder. The resultant gaseous chlorides (iodide) envelopes the object being plated, saturating the external layer with chromium. It becomes bonded with the basic material much more firmly than when the chrome plating is done galvanically.
Chemists in Lithuania have developed a process of creating a multi-layer “coat of mail” for especially vital components. The extremely thin outward layer of this covering (and it does look like mail under the microscope) is chromium. During service it is the first to be attacked by oxygen but years will pass before it will become oxidized and all this time the component it covers will be doing its important duty.
Until recently only metal parts were chrome-plated. But now Soviet scientists have developed a process of chrome-plating plastics. When tested, chrome-plated polystyrene, the well-known polymer, proved to be much stronger and was not affected by some of the worst of the known “enemies” of structural materials – wear, bending fatigue, and impact. And the service life of the parts made from this material has naturally become longer.
The chrome “armour” has been proved useful even for diamonds that are themselves rightly considered the standard of hardness. It appears that not all the diamonds mined can be used in tools. As a rule, narual diamonds have a patina of tiny cracks, making it impossible to use them in cutters or bits: as soon as such diamonds come into small splinters or crumble out of their holders. Scientists have suggested that the diamonds should be protected by a thin coating of chromium which bonds well both with the diamonds and the copper holders.
The metal-coated diamond was tested and it was proved that it was not only secure in the holder, but the service life of the crystal itself became several times longer. A microscopic examination of the crystal revealed that one of the facets had quite a deep crack “cemented” by the film covering the diamond. It became clear that the atoms of chromium combining with the diamond’s carbon had formed hard carbides on its surface and that chromium had penetrated into the crack whose walls became covered with carbide “armour”. Meanwhile the layer of pure chromium that was in contact with the holder had fused with copper, owing to which the diamond became firmly fixed in the tool. Thus, chromium made it possible to “kill two birds with one stone”: the tool became more durable and the diamond became harder than…dimamond.
…Before winding up our story about chromium, we would like to quote V.S. Yemelyanov once more. In 1967 he wrote: “Two years or so ago I heard a news which, alas, passes unnoticed in our country. We had sold a consignment of ferrochromium to Britain, a country which we had always taken for a symbol of technological progress. And now Britain buys our ferrochromium! And the British know what they are doing.”
Source: Tales About Metals, S. Venetsky